I'lioroMicKoGU.U'iiy .ixi> rnciiMC.ii. mickoscui'V \n 



Mk R(>\ll■:rRV 

 Mi^r^)HU•try |>l>i>s ,111 iinpiiitaiit part in tiiliiiir.il tiiiirostopy 

 iM-caus*' tlu- ilinu'iisions of miiTo-ronstitiR'nts in a sj)ccinuMi are lu-lprul 

 for purpose's of identification or for forecasting physical projiertics. 

 In mctallojjraphy the measurement of grain size is assuming importance 

 for certain alloys and in some casi-s, specifications are so drafted as 

 to define this characteristic. 



For measuring objects under the microscope, the e\epiece contains 

 a glass disc on which fine divisions have been ruled. In some cases, 

 these rulings take the form of a cross-section composed of small 

 squares or rectangles. The reading of each division of the eyepiece 

 micrometer is calibrated for each objective b>- comparison with a 

 stanilardized stage micrometer. These stage micrometers are glass 

 microscope slides on which known units of length ha\e been accur- 

 atel\- ruled, such as 1 mm. di\ided into one hundred iiarts or 3 nmi. 

 di\ided into tenths and one-tenth divided in hundredths of a nmi., 

 etc. The stage micrometer is focused in the same way as a micro- 

 scopic sju'cimen and adjusted into position so that the rulings of the 

 eyepiece micrometer arc superimposed on them. It is then possible to 

 evaluate the eyepiece rulings in terms of the standardized stage 

 micrometer, after which the latter is removed and the specimen to be 

 examined substituted in its place. Thereafter, the image of the eye- 

 piece rulings will be superimposed on the image of the specimen and 

 measurement can proceed. For precision work, a very accurately- 

 made eyepiece micrometer is used, a typical form of which has a 

 thin glass plate upon which is ruled a cross and a double line. This 

 is mounted on a slide immediately below a stationary micrometer 

 scale and can be moved by means of micrometer screw. The cross 

 is accurately set by the micrometer screw to coincide with the par- 

 ticle to he measured, the double line serves to count complete revolu- 

 tions of the screw with the aid of the scale which is seen in the field 

 of vision. The screw carries a drum which has .50 divisions and 

 each division corresponds to a displacement of the cross through a 

 distance of 0.01 mm. so that a complete revolution of the drum dis- 

 places the cross 0..50 mm. The actual readings of each interval of 

 the drum head must be accurately calibrated for each objective by 

 means of a stage micrometer. A group of instruments for use in 

 micrometry is illustrated in Fig. 2(5. 



Applications of Photomkrocirai'iiy 



In closing, attention is directed to the photomicrographs comprising 

 the Appendix of this paper, each of which was taken in connection with 



